Lithofacies and diagenetic controls on thrombolitic dolomite reservoir development from the Precambrian Dengying Formation in the central Sichuan Basin, SW China
Yong LiBenjian ZhangGang ZhouRuifeng TangHongyu LongChenglong LiXi ChenDi ChenXiaohong LiuChao ZhangMingyou Feng
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Microbialite reservoirs are of great importance in oil and gas exploration. However, there is still a lack of comprehensive studies on the formation mechanisms of thrombolitic reservoirs, a specific type of microbialite. This research focuses on the oldest thrombolitic dolomite reservoir located within the Precambrian Dengying Formation in the central Sichuan Basin, southwestern China. A multi-disciplinary approach was employed to characterize different thrombolite facies and elucidate the formation mechanism of thrombolitic dolomite reservoirs and their controlling factors, involving core observation, thin-section analysis, cathodoluminescence, scanning electron microscope (SEM) microscopy, elemental analysis using LA-ICP-MS, and carbon and oxygen stable isotope analysis. Based on variations in texture, four types of thrombolite were identified: 1) distinct clotted thrombolite, 2) diffuse and regular clotted thrombolite, 3) diffuse and irregular clotted thrombolite, and 4) composite clotted thrombolite. Notably, the diffuse clotted thrombolitic dolomite is the prevalent lithology in the reservoir. Through modification by meteoric water, organic acid fluids, and hydrothermal fluids, a reservoir with dominant porosity in the form of primary growth-framework pores, dissolution pores, and vugs was formed. This resulted in the development of two high-quality reservoir intervals within the Second Member and at the top of the Fourth Member of the Dengying Formation. The growth-framework porosity of the thrombolites, epigenetic karstification, and tectonic fracturing were mainly conducive to reservoir development. However, various types of cementation have reduced porosity and connectivity within the reservoir. Overall, this study is a valuable example of the methodology required to understand meso- and microstructures of deep-buried thrombolitic dolomite reservoirs, including their heterogeneities and diagenesis, as the original structures influence diagenesis.Keywords:
Sichuan basin
Dolomitization
Abstract We clarified three stages of dolomitization and secondary changes by studying the petrology and geochemistry characteristics of dolomite from the Ma5 5 –Ma5 10 sub‐members of the Ordovician Majiagou Formation in the Jingxi area in the Ordos Basin: (1) Syngenetic microbial dolomitization is characterized by formation of dolomite with a mainly micrite structure and horse tooth‐shape dolomite cements. (2) Seepage reflux dolomitization during the penecontemporaneous period superposed adjustment functions such as recrystallization and stabilization in the middle‐deep burial stage, forming dolomites mainly consisting of micro crystal and powder crystal structure. (3) Powder dolomite, fine dolomite, and medium‐coarse crystalline dolomite formed in pores and fractures in the middle‐deep burial stage. The secondary concussive transgression‐regression under a regressive background is an important condition for the occurrence of many stages of dolomitization in the study area. The basin was an occlusive epicontinental sea environment in the Ma5 member of the Ordovician Majiagou Formation sedimentary period. In the sediments, sulfate content was high, which is conducive to the preservation of microbial activity and microbial dolomitization. Micritic dolomite formed by microbial dolomitization provides good migration pathways for seepage reflux dolomitization. Affected by evaporation seawater with increased Mg/Ca ratio, seepage reflux dolomitization was widely developed and formed large‐scale dolomite, and underwater uplifts and slopes are favorable areas for dolomite. In the middle‐deep burial stage, dolomitizing fluid in the stratum recrystallized or stabilized the previous dolomite and formed a small amount of euhedral dolomite in the pores and fractures.
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Dolostone
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Aiming at the scientific problem that only part of dolomite acts as dolomite reservoir, this paper takes the multiple dolomite-bearing formations in the Tarim and Ordos basins, NW China and Sichuan Basin, SW China as the study object, by means of mineral petrological analysis and geochemical methods including carbonate clumped isotope, U-Pb isotopic dating, etc., to rebuild the dolomitization pathway and evaluate its effects on reservoir formation. On the basis of detailed rock thin section observation, five dolomitic structural components are identified, including original fabric-retained dolomite (microbial and/or micrite structure), buried metasomatic dolomite I (subhedral–euhedral fine, medium and coarse crystalline structure), buried metasomatic dolomite II (allotriomorphic–subhedral fine, medium and coarse crystalline structure), buried precipitation dolomite and coarse crystalline saddle dolomite. Among them, the first three exist in the form of rocks, the latter two occur as dolomite minerals filling in pores and fractures. The corresponding petrological and geochemical identification templates for them are established. Based on the identification of the five dolomitic structural components, six dolomitization pathways for three types of reservoirs (preserved dolomite, reworked dolomite and limestone buried dolomitization) are distinguished. The initial porosity of the original rock before dolomitization and the dolomitization pathway are the main factors controlling the development of dolomite reservoirs. The preserved dolomite and reworked dolomite types have the most favorable dolomitization pathway for reservoir formation, and are large scale and controlled by sedimentary facies in development and distribution, making them the first choices for oil and gas exploration in deep carbonate formations.
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Carbonate minerals
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Based on petrology and mineralogy, the geochemical characteristics of dolomite in the Cambrian Longwangmiao Formation in the eastern Sichuan Basin, were carried out. Results showed that dolomites mainly occurred in the middle and upper parts of Longwangmiao Formation, with most commonly as a crystalline dolomite and less commonly as a gain dolomite; the percentage content of dolostone in dolomite ranged from 48.6% to 75%, ratios of Mg/Ca were generally small, and positive and negative relationships between content of MgO and content of CaO were identified, respectively; the dolostones were generally characterized by high contents of Fe and Na while low contents of Sr and Mn, and only a part of dolostones had high content of Mn; compared with composition of contemporaneous seawater, most of dolostones were characterized by positive δ13C values and negative δ18O values. Based on the analyses of regional geological setting and geochemical characteristics, the dolomite in Longwangmiao Formation was mainly from the seepage-refluxion dolomitization, while evaporative-concentration dolomitization and burial dolomitization were not developed. In general, the dolomitization of Longwangmiao Formation was not complete. Furthermore, through the dolomitization model of Longwangmiao Formation, the distribution of sedimentary facies as well as the controlling effect of dolomitization on reservoir, the favorable reservoir was predicted, and was mainly developed around Qiyueshan fault zone.
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Dolomitization
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Increasingly significant proportions of the hydrocarbons all over the world,including China,are found in dolomite reservoirs.Nevertheless,in despite of more than 200 years of efforts,the dolomite problem-the geneses of dolomite(Warren,2000) still remains disputable.Most critical observations and conclusions available are: ① evidences revealed that the dolomite mostly formed by secondary replacement,i.e.dolomitization processes,② it is kinetic factors rather than thermodynamics that restrain the lower temperature syntheses of dolomite precipitates,③ the sources and the volume of Mg-rich fluids control the extent and scale of dolomitization,and ④ microbial activities may be another important factor of favoring the formation of dolomite.To explain different forms of dolomite,many models are proposed in literatures,focusing on the conditions and the locations of the interaction of the Mg-rich fluids with the preexistent calcareous carbonates.Generally speaking,condensed marine water dolomitization,Sabkha model,mixing-zone dolomitization and burial adjustment are relatively of great importance.
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Summary Stratigraphically discordant dolomite bodies have long been studied and documented on the Arabian Shelf. Dolomite mapping as a first step towards a carbonate diagenetic modelling, is critical for understanding the dolomitization mechanism, and reservoir quality prediction. Subsurface dolomite mapping is a well-known challenge due to subtle difference between limestone and dolomite, but the better understanding of the dolomitization process could provide an unexpected approach to solve this problem. An innovative workflow is developed to map the dolomite distribution driven by the diagenetic mechanism and comprehensively characterize fracture systems in a multiscale way. The mapping results show that massive dolomite bodies are heterogeneously developed and distributed mainly in the northeastern part of the study area, with a southward decrease in dolomite content. The results clearly demonstrate that multiscale fracture systems play critical roles in the massive dolomitization, which provides new insights on the dolomitization mechanism and subsurface reservoir and seal.
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Lower Ordovician dolomites in the central Tarim basin are of various types and extensively distributed.Seven dolostone types could be documented based on fabrics and structures,which are crystalline dolomite,residual-grain dolomite,residual calcite dolomite,algal dolomite,cloudy centers and clear rims dolomite,clitellum dolomite and gypes dolomite.It's characterised by middle-high δ18O negative value(-3.2%~-8.7‰),low-middle δ13C negative value(-3%~-0.77‰),and high Z value of 118~123.Most diagenetic temperature calculated by empirical formulas using δ18O values are between 118 to 123℃,which indicates that the temperature degree of dolomitization is low,and so the kind of dolomite belongs to low temperature dolostone.These dolomites have also characteristics of low Mg/Ca and variable Fe/Mn ratio values.According to mineral composition analyzed by electron microprobe,dolomite monomineral can be divided into high Fe content and low Fe content species.87Sr/86Sr ratios of most samples are low(among 0.708 8~0.709 7),which are similar to seawater strontium isotope composition,and some are of high ratios between 0.710 1 and 0.710 9.These characteristics reveal that the origin of dolomite in the study area can be classified into penecontemporaneous dolomitization and burial dolomitization.According to Ordovican palaeogeographical evolution,the penecontemporaneous dolomitization arised in the seafloor environment,mainly in tidal flats or beach environment.And this kind of dolomite characterised by low diagenetic temperature,low 87Sr/86Sr ratios,and low Fe content dolomite minerals.The burial dolomitization can be interpreted by two diagentic models.One is seepage-reflux dolomitization during early shallow burial stage and characterised by low diagenetic temperature but high 87Sr/86Sr ratios,and high Fe content dolomite mineral.Another is deep burial dolomitization which occured in more strong reducible diagenetic enviornment during late deep burial stage.In general,the degree of dolomitization in burial diagentic enviorment is further enhanced and adjusted.The deep burial dolomite is characterised by high diagenetic temperature and high homogenization temperature of fluid inclusions,but low salinity.
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Dolomitization of the Devonian Jefferson Formation in south-central Montana probably occurred as a result of multiple dolomitizing events. Petrographic and geochemical studies show the presence of five distinct dolomite types: cathodoluminescently nonzoned euhedral dolomite, nonzoned subhedral/anhedral dolomite mosaics, zoned euhedral dolomite, irregularly luminescent dolomite, and zoned dolomite cement. Data from Jefferson Formation dolomite indicate a progression from early, nonstoichiometric, euhedral replacive dolomites, to nearly stoichiometric, subhedral/anhedral dolomite mosaics. {delta}{sup 18}O and {delta}{sup 13}C ratios decrease with increasing degree of dolomitization. Euhedral replacive dolomites in lower dolomitic limestone layers probably formed by early, refluxing hypersaline brines in evaporitic settings. These dolomites are the least stoichiometric and have the heaviest {delta}{sup 18}O signatures of all dolomites which were analyzed. Massive dolomitization of the upper two-thirds of the Jefferson Formation probably occurred when dolomitizing brines migrated downward through the upper Jefferson Formation during deposition of thick evaporite sequences in the overlying Three Forks Formation. Mass balance calculations indicate that this mechanism was more than adequate to cause the massive dolomitization in the upper Jefferson Formation. Massive dolomitization during Logan Gulch deposition may have been the final phase of dolomitization, or dolomite may have recrystallized to more stoichiometric, {sup 18}O- and {sup 13}C-depleted subhedral/anhedral more » mosaics during later burial. « less
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